Re: [linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging

[Mikulas Patocka <mpatocka redhat com>]

On Wed, 4 Jun 2008, Steeve McCauley wrote:

> Is this change meant to add cluster support for merging?  I had
> been under the assumption that merging was already a feature of
> snapshots.

My implementation has nothing to do with clustering.

I think there were some userspace programs for off-line merging of 
inactive snapshots. My implementation works in kernel and can be used 
while the snapshot is mounted.

> What is the difference between the current implementation using
> lvremove (which I assumed was where the COW blocks were merged back
> into the original volume) and lvconvert as below?  Why was this
> functionality not kept in lvremove?

lvremove drops the snapshot and forgets its content. lvconvert -M copies 
the snapshot to the origin (I'm now working on to let it automatically 
drop the snapshot when the merging finishes).

Mikulas

> Mikulas Patocka wrote:
> > Hi
> >
> > Here I release the first experimental implementation of snapshot merging. 
> > Merging allows you to copy data in snapshot back to the origin device. Once 
> > merging starts, it runs on background. When the merging finishes (you see 
> > "0%" with "lvs" command), you should remove the merging snapshot with 
> > lvremove command.
> >
> > The userspace will still be changed, the kernel is already finished (there 
> > are no known bugs in the kernel, I'm curious about the unknown ones :)
> >
> > Merging is initiated with "lvconvert -M vg/lv_snapshot" command. When you 
> > initiate merging, make sure that the origin device is not mounted. Snapshot 
> > can be mounted while it's merging is initiated.
> >
> > During merging, aby reads and writes to the origin device are identical to 
> > accesses to the merging snapshots.
> >
> > There may be multiple snapshots while one of them is being merged --- 
> > exceptions in other snapshots are being allocated and there snapshots are 
> > kept stable.
> >
> > Don't try to concurrently merge more than one snapshot (the kernel refuses 
> > to do it, the userspace tools don't know about it --- the userspace will be 
> > rewritten anyway).
> >
> > The patches are at:
> > http://people.redhat.com/mpatocka/patches/
> > The patches are for 2.6.26-rc4
> >
> > (besides merging, there are some other bugfixes, I made for the kernel)
> >
> > Mikulas
> >
> > _______________________________________________
> > linux-lvm mailing list
> > linux-lvm redhat com
> > https://www.redhat.com/mailman/listinfo/linux-lvm
> > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
> --
> Steeve McCauley
> Director of Research and Development
> Rackable Systems
> 407 McGill Street, Suite 1002
> Montreal, Quebec, H2Y 2G3 (Canada)
> Tel: 514-288-3343
> http://www.rackable.com
> :wq
>
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